Wireless devices or mobile stations (MS) such as cellular handsets transmit and receive speech waveforms. Baseband signal processing internal to such a handset is generally divided into two principal components, a vocoder and a modem. The function of the vocoder is to perform source encoding and decoding on speech waveforms. Source coding removes redundancy from the waveform and reduces the bandwidth (or equivalently the bit-rate) in order to transmit the waveform in real-time. The modem typically includes a channel encoder/decoder and a channel modulator/demodulator. The function of the channel coder is to increase redundancy in the transmitted signal in a controlled fashion to enhance the robustness of the transmitted signal. The channel modulator/demodulator converts the data stream into a format suitable for transmission in a bandlimited radio frequency (RF) channel.
When a mobile station is in transit during use, reception between the mobile station and a serving cell such as a base station can degrade. For example, the RF signal may fade as the mobile station moves farther away from a serving base station. Accordingly, a handover process may occur, in which serving of a mobile station transfers from a first serving cell to a second serving cell. During the handover, various procedures are performed to transfer the mobile station between serving cells. These procedures allow the mobile station to be synchronized to the new serving cell, which may operate at a different frequency and different timing parameters, for example, different hypersequences and the like.
Handovers are among the most complex functions that a wireless network manages, and accordingly challenges exist in maintaining a high quality connection during such a handover. To effect a smooth transition, some mobile devices simply mute audio during the handover process and start up fresh on the new serving cell. However, this mute period may be sufficiently long to disturb the caller who may believe that the telephone call has been terminated. Thus other phones maintain the audio path during the handover and pad the gap between serving by the first cell and the second cell with a noise fill, e.g., a comfort noise that a user perceives as background noise from the uplink device. Accordingly, the user more readily understands that the telephone call is continuing. However, this maintenance of the audio path greatly complicates synchronization between the new serving cell and components of the mobile station.
Further complicating the issue in some phones is a so-called time domain isolation (TDI). Such TDI provides for periods of gating off of noisy digital circuitry of a phone when a radio portion is operating, leading to challenges in managing a codec interface. Because of TDI, relatively large buffers for audio data exist in hardware of the mobile station. Specifically, both uplink and downlink audio paths incorporate large buffers that can hold significant amounts of data, e.g., at least 10-15 milliseconds (ms) of audio data. These buffers further complicate handovers.
A need thus exists to improve handing over of a mobile station from one serving cell to another.